Nitrogen fixation, soil quality and restoration trajectories in agricultural matrices of lowland Canterbury, New Zealand

The aim of the present study was to investigate the relationships between nitrogen (N)-fixing plants, associated symbiotic bacteria and soil properties, and to evaluate the ecological role of native N-fixing plants in the context of ecological restoration in agriculture landscapes of New Zealand. The work had a particular focus on a restoration project associated with a plantation forest to farmland conversion at Eyrewell in Canterbury. Approximately 150 ha has been set aside for ecological restoration, with an additional 150 ha of native plants being established on paddock and farm borders. The original dryland vegetation of nutrient-poor acidic soils is being restored and embedded within an intensively irrigated and fertilized agricultural matrix. A paucity of knowledge of the functional role of native N-fixing plants in New Zealand plant communities is probably surpassed by research addressing the widespread weed problem of invasive exotic N-fixing gorse and brooms. Five endemic species of the Leguminosae and Rhamnaceae, and three exotic species of Leguminosae were investigated. Laboratory experiments were carried out initially to isolate and identify N-fixing bacteria, with additional access to existing collections. N-fixing bacteria were then inoculated to native and exotic legumes in glasshouse experiments. Viable cultures of Frankia associated with Discaria were difficult to isolate and culture. A locally rare early-successional endemic species of non N-fixing plant was included in the study as a reference plant. Nitrogen, phosphorus and lime amendments were added to soils in a glasshouse pot experiment. A fertilizer trial was carried out in the field at Eyrewell. Native N-fixing species, associated assemblages of plants and soils in more natural plant communities in field in the Canterbury region were also located and described. The results showed that inoculation of N-fixing bacteria on legumes improved plant growth and nodulation but this varied according to species and plants’ age. Native N-fixing species were tolerant of but not responsive to high nitrogen agricultural soils. Urea fertilizer application led to increased soil acidity and phosphorus improved plant nodulation. There were some evidences that native species are adapted to New Zealand’s acidic soils. Native N-fixing plants are able to improve the growth of other native plants, and maintain or increase available nitrogen in soil. This was quantified and the amounts were shown to be significant. Native N-fixers were found to naturally occur within plant communities that support a large number of other native species. Experimental research showed they may contribute a different and more beneficial role than exotic legumes in diverse native plant communities. The findings of this research project indicated that N-fixing plants should be considered as an essential component of the restoration matrix in the ecologically-degraded landscapes of Canterbury and probably more widely in New Zealand. This research project has provided new insights into the interaction between N-fixing bacteria, N-fixing plants and soil properties, and the role of native N-fixers to restoration in agriculture landscapes.